Deere's Cyclone engines were introduced for the 47 model year update, along with the change to pressed steel frames and the introduction of gasoline only engine options (as opposed to the previous all-fuel offerings). IIRC the basic idea behind development of the Cyclone engine was that adding "eyebrows" around the intake valves would cause the intake charge to swirl and lead to more thorough mixing of fuel and air, resulting in more complete combustion and therefore more power. Additionally, an increase in the compression ratio coincided with the introduction of gasoline only engines. I know the gas burners from 47 through the end of letter series production had the cyclone engine, but I don't remember if all-fuel models benefitted from cyclone technology or not. Cyclone engines do produce significantly more power than their non cyclone counterparts.
FWIW, Dad collects old tractors and currently has 22, some restored, many unrestored, but all are in operating/working condition. He has 2 A's, a 36 & 45 model, neither with the cyclone motor, and 2 B's -- 36 &51 models. The 51 B has a Cyclone motor, and I believe the Nebraska test data of the B with a Cyclone engine very close to the same hp as the A's without the Cyclone engine.
